Yeah, that 50/50 thing isn't true; yesterday I tried to take back a planet with about 2800 combined attack/defense after Legion bonus, I had over 4200 invasion attack, and my odds were 39%.
EDIT: Didn't notice this post before--
KarlLitos wrote:
The planet get legion bonus, and more att/def if someone guard it.
Your mutagenic cartriges only give you 1500 more invasion attack, make some math.
According to what he said, a full Legion bonus would give that planet about 3200 total invasion defense, whereas he had over 3600 attack, so there doesn't seem to be anything wrong with his statement. And guards won't affect it because you can't invade a guarded planet anyway.